1. Field of the Invention
The present invention relates to an image sensing apparatus and an imaging system.
2. Description of the Related Art
Conventionally, to improve the frame rate at which signals are read out from the pixel array of an image sensing apparatus, sub-sampling readout has often been adopted. In the sub-sampling readout, signals are read out from the pixels of not all rows but some rows in the pixel array. When this image sensing apparatus performs the sub-sampling readout, light enters not only readout rows from which signals are read out but also non-readout rows from which no signals are read out. There are cases where the pixels of the non-readout rows adjacent to the readout rows often photoelectrically convert electric charges in an amount that exceeds the saturated charge amount upon light reception so that these electric charges overflow into the readout rows. In this case, problems such as smear and blooming in which false signals mix into signals read out from the pixel array are posed, which may degrade the quality of the obtained image.
To solve this problem, Japanese Patent Laid-Open No. 2000-350103 discloses a technique of resetting the pixels of the non-readout rows in at least part of the period during which the pixels of the readout rows accumulate electric charges. This makes it possible to suppress electric charges in an amount that exceeds the saturated charge amount from overflowing from the pixels of the non-readout rows into those of the readout rows, thus suppressing degradation in the quality of the obtained image.
The technique disclosed in Japanese Patent Laid-Open No. 2000-350103 resets the pixels of all non-readout rows. With this operation, an unnecessarily large current flows in the image sensing apparatus. This may increase power consumption more than necessary upon reading out signals from the pixel array of the image sensing apparatus.